Magnetohydrodynamic studies in the FTU

The main magnetohydrodynamic (MHD) activities affecting the Frascati Tokamak Upgrade (FTU) high-field plasmas with limiter configuration are sawtooth relaxations and tearing modes. The period of sawtooth relaxations can be increased in FTU both by electron heating and by pellet particle deposition near the sawtooth inversion radius; both methods lead to full stabilization in proper conditions. The sawtooth period can be shortened as well by central heating. The influence of localized electron cyclotron resonance heating (ECRH) on the stability of m = 2 tearing modes has been studied in FTU by means of radial and power scans. Heating between the plasma center and the island location increases the island size, while heating at the island location produces mode stabilization if ECRH power exceeds a threshold value. These sawtooth and tearing mode studies show that some control of both phenomena can be achieved. Double-tearing modes in the form of regular, sawtooth-like relaxations have been observed in discharges with reversed magnetic shear. The development of these instabilities is particularly interesting in FTU as it happens in the absence of injected momentum. Long-lived m = 1 island structures are frequently observed following pellet deposition near the inversion radius; particle accumulation around the O-point enhances diagnostic sensitivity, thus allowing fine studies of island dynamics. MHD spectroscopy has revealed the existence of coherent waves at frequencies well above the drift-tearing range in thermal plasmas. In addition, broadband turbulence has been observed both in ohmic and in radio-frequency-heated plasmas. The amplitude of turbulent fluctuations increases with heating power and is anticorrelated with the neutron yield.